Page 50 - Earth's Climate Past and Future

26 PART I • Framework of Climate Science

Coastal ocean FIGURE 2-10 Climate resolution
Continental and Deep The degree of resolution of climate
lakes shallow seas ocean records in sediment archives is related
to the rate of deposition (and burial)
Typical of sediment and to the amount of
sediment 1 mm 10 cm–1 m 1 cm activity of organisms burrowing into
influx rates per year per per the sediments.
1000 years 1000 years

1–10 cm 10 cm–1 m 1–10 cm
Typical
depth of
mixing

Typical
resolvable 10–100 100–1000 1000–5000
years
years
detail years

Typical rates of sediment deposition range from as cores from the ice sheet on Greenland, where deposition
much as meters per year in coastal marine sequences of snow is rapid, the annual layering may remain detectible
and millimeters per year in lakes to millimeters per tens of thousands of years into the past. In the polar ice
thousand years in some deep-sea sediments. Rates can sheet covering eastern Antarctica, where only a small
vary locally around these average values by a factor of amount of snow accumulates each year, annual layering
10 because of factors such as the amount of sediment may not occur even at the ice surface.
supplied locally by rivers or redistributed by currents. Tree Rings and Corals At middle and high lati-
The degree of disturbance by organisms that move tudes where trees produce annual layers, tree rings
across and burrow into the sediment surface also varies become a permanent record of annual climate change
with environment. In highly productive coastal regions, unless they are later disturbed by fire or by sporadic bor-
large organisms burrow tens of centimeters or even meters ing by insects or excavation by birds. Similarly, CaCO
3
down into the sediment. Relatively unproductive deep bands in corals form a permanent record of seasonal to
ocean basins have fewer and smaller bottom-dwelling annual climate change.
organisms that typically burrow down no more than a few The types of climate archives, the maximum time
centimeters. Most lakes also have fewer and shallower span of the records they contain, and the highest resolu-
burrowers. As a result, the resolution of sedimentary tion achievable in each archive are summarized in
records varies with environment. Lakes usually have the Figure 2–11 in a log time scale that changes by powers
best resolution and deep-ocean sediments the poorest, of 10. Also shown at the top are the time spans covered
although locally rapid deposition can improve resolution by the major parts of this book.
in some ocean areas.
After particles pass through the upper layers, no fur- Climatic Data
ther mixing occurs unless erosion reexposes the sequence
back at the sediment-water interface. Increased pressure Climate archives contain many indicators of past climate
and loss of water caused by deep burial of sediments referred to as climate proxies. Climate scientists use the
gradually compact the sediment layers and turn them term “proxy” (meaning “substitute”) because the process
into soft rock, but do not dramatically reduce the resolu- of extracting climate signals from these indicators is not
tion they can provide. direct, like reading temperature from a thermometer.
Ice Cores Annual layers of snow are visible at the Instead, scientists must first determine the mechanism
surfaces of many mountain glaciers and rapidly deposited by which climate signals are recorded by the proxy indi-
ice sheets (see Figure 2–9A). As the snow is buried and cators in order to decipher the climate changes. (Of
slowly recrystallized into ice, annual layers remain resolv- course, even a typical thermometer relies on a “proxy”
able to a depth that depends on their initial thickness at the measurement—the height of a column of mercury cali-
time of deposition. Below this level, the layering is lost. In brated to indicate temperature.)